It's more like joining a class action settlement.

Your lawyer is your agent. A lawyer representing a group of which you are a part is representing the group, and your interests and the groups can diverge.

That divergence can happen with a personal lawyer, too: https://en.wikipedia.org/wiki/Principal–agent_problem

The reason an employee may want to join a union is because this divergence has already happened between the employee and the employer.

The union is also a worker’s agent. It’s monitoring problems all the way down.